We in Northern California are accustomed to a fair share of severe thunderstorm activity during the cold season (especially December through April). Mini-supercells occur every year in the Central Valley of California, usually in a cold and unstable post frontal atmosphere or on the east side of a strong low pressure area (diffluence aloft; decent influx of moisture from the south). In the past decade there have been some memorable mesocyclonic and non-mesocyclonic tornadic events in CA, contrary to popular belief. The most recent...and best documented...event occurred in 2005 near the state capitol of Sacramento. Some of the tornados were very impressive but traversed mainly unpopulated agricultural areas, but a couple of them caused some damage near Natomas (F0-F1). John Monteverdi of San Francisco State University has done some interesting work on the elusive California tornado, and a summary is available at
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http://tornado.sfsu.edu/SacTornado/SacTornado.html >
South San Francisco, which had never had a tornado in recorded history, experienced a damaging F1 this winter similar (or even a little more robust) than the Sydney one in the video.
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http://www.wrh.noaa.gov/mtr/ssftor_pns.php >
Considering that many of California's cities average only one or two days per year on which thunder is audible (which barely counts, I say!), the number of tornados is rather suprising. Some have suggested a mini-Tornado Alley effect--that the gradually sloping terrain of the East Valley is somewhat similar to the Great Plains and therefore more conducive to large-scale forcing and condensation. There are certainly some parallels between the cold season convective activity in southeastern AU and the Pacific Coast of the US, though tunderstorms are generally much more prevalent in Sydney than San Francisco.
That is, in any case, quite the video.
